This invention relates to incident-light phase grids, and particularly to incident-light phase grids suitable for use with a position measuring instrument.
Incident-light phase grids are used for example in photoelectric position measuring systems for measuring the relative position of two movable objects. The objects movable relative to one another may consist of two machine components of a processing machine. The phase grid can be used as a measurement embodiment in such a system.
An incident-light phase grid serves to create a desired phase displacement in light reflected off of the phase grid. That is, lightwaves of equal wavelength proceeding from adjacent grid elements always have the same, predetermined phase difference. This phase difference is created by the difference in optical path length between the two light paths, one of which is reflected off an upper reflection layer and the other which is reflected off a lower reflection layer included in the step grid of the incident-light phase grid. Such step grids are defined with a predetermined step depth in order to obtain the desired phase difference.
The paper of J. Wilhlem entitled "Dreigitterschrittgeber" (1978, pages 19 and 20) describes a phase grid composed of two amplitude grids mounted with a corresponding displacement. In this system, under one amplitude grid there is additionally laid a phase layer.